1. Field of the Invention
This invention relates generally to detecting faults in multipair cables and, more particularly, to a method and apparatus for indicating which pairs in the cables have transpositions or reversals.
2. Description of the Prior Art
In the subscriber loop portion of telecommunications systems, customer service is generally provided over electrical cables comprising many individually insulated conductors. These conductors are twisted together into pairs and a large number of the pairs are contained within a single protective sheath which includes a continuous metallic layer. The individual leads of the conductor pairs are designated the tip and ring wires. Each of the twisted pairs typically connects the customer premises to a switching point, usually a central office.
In providing a new connection between the customer and the switching point (or indeed between switching points), it is necessary to splice together a number of shorter cable lengths to establish the overall cable route. Also, in old installations, it oftentimes becomes necessary to replace one or more of the shorter cables or to divert or extend a cable route by splicing in a new cable section or path. In all these situations, to insure reliable completion of the splicing operation, testing for defective pairs is effectuated after the splicing activity.
During the splicing work, conductor pairs may be rendered defective in a number of ways. A conductor may be inadvertently broken or a conductor pair may be shorted or have a low resistance path created between the wires. These types of faults preclude using the pair because of the service-affecting nature of the fault. Fortuitously, however, these faults may be readily detected at a single location by suitable resistance or capacitance measuring devices long available in the art.
Particularly troublesome, though, are those faults that require a two-ended testing operation for reliable detection. For instance, in many applications, such as multiparty or TOUCH-TONE phone service, it is necessary to maintain proper tip-ring orientation throughout the entire cable route. Loss of tip-ring orientation results in a fault condition called a reversal. Since this fault results from mere criss-crossing of wires within a pair, it is virtually impossible to detect a reversal using a single-ended detection scheme. Of necessity, double-ended testing is prescribed, and the instant disclosure is directed towards the detection of reversals utilizing a method and apparatus simultaneously accessing both ends of the cable route undergoing troubleshooting.
Another fault that precludes single-ended diagnosis is the so-called transposition. Like the reversal, this fault involves the criss-crossing of wires, but in such a manner that both wires comprising one pair are interchanged with those of another pair. Again, conventional, single-location testing techniques are rendered virtually useless in troubleshooting a transposition. The instant disclosure is also directed towards the detection of transpositions.
A number of two-ended testing arrangements are known from prior art disclosures. They range: from the most rudimentary fault detecting techniques wherein a test signal is manually placed on one pair at a time and probing means are used to search through a bundle of pairs for the pair at another location; to the more sophisticated fault detection types wherein synchronized equipment sequences rapidly through a multitude of pairs while measuring for fault conditions.
U.S. Pat. No. 3,986,106, issued to E. E. Shuck et al on Oct. 12, 1976, is representative of the latter type arrangements. Shuck et al employ sets of precision, matched resistors to detect faults including reversals and transpositions. Control and slave sets, one at each end of the cable run, are automatically sequenced in fixed intervals. During the interval that a particular pair is addressed, a generator in the control set transmits a periodic pulse both onto the pair under test and through a corresponding path in the set. Detection circuitry compares: the pulse returned from the pair having a precision terminating resistor placed on the pair by the slave set; with the same pulse sent internally through the other correspondingly matched resistor. If the comparator determines the returned pulse magnitudes are equal, sequencing continues. Otherwise, a fault condition obtains and the number of the faulty pair is indicated via a display.
A number of shortcomings are inherent with techniques of this type. A major shortcoming is one that limits the maximum number of pairs that can be connected to the equipment at one time. To effectively determine faults, each cable pair is assigned a different resistance so that any short, open, misconnection, cross-connection and so forth will be detected by the difference in compared pulse amplitudes. However, as more pairs are connected to the apparatus, the gap between resistor values diminishes. Ultimately, normal cable parameters, such as leakage resistance, as well as parameter variations and noise considerations set an upper bound on the number of pairs that can be simultaneously connected. Another difficulty resides in the use of a periodically pulsed drive signal. It is probable, particularly when the splicing activity involves previously working pairs, that a ringer circuit on a bridged tap (a secondary cable path served by the primary route) may distort the return pulse and thereby falsely indicate a fault. Yet another limitation involves the use of matched, precision resistors in the harsh outside plant environment. In many applications, the control set may be located in the field whereas the slave set is housed in the central office. Any significant temperature disparity may generate testing errors.
U.S. Pat. No. 4,015,200, issued to M. T. Strandh on Mar. 29, 1977, also discloses a two-ended, sequential testing arrangement, but for the purpose of testing individual conductors rather than conductors grouped as pairs. The apparatus can only be used to detect an incorrect connection of individual wire pairs and cannot distinguish between pair transpositions and reversals.
U.S. Pat. No. 4,074,187, issued to D. H. Miller et al on Feb. 14, 1978, discloses two-ended testing equipment to detect, among other faults, transpositions and reversals on a multipair cable wherein each pair is individually shielded. The apparatus comprises a plurality of light-emitting diodes (six per pair) and switches to test for the various fault conditions. Two LEDs per pair are in the remote test unit, thereby requiring a craftsperson to be present at the remote location to interpret the display.